All 3rd-generation (3G) mobile radio systems which are relevant for the mass market operate using the (full) frequency duplex mode FDD. That is to say, transmission and reception take place on different frequency channels. The transmission methods which are used in this case are virtually exclusively code division multiple access methods CDMA, which require “real” simultaneous transmission and reception by the telephones and the base stations. By way of example, the mobile radio standards CDMA2000 and W-CDMA (UMTS) in Band II (1.9 GHz) and W-CDMA (UMTS) in Band I (2.1 GHz) may be mentioned here. These are used, inter alia, on the American and European continents.
A frequency duplex mode such as the CDMA method requires duplexers which permanently connect both the transmission path (TX) and the reception path (RX) to the antenna AN, see FIG. 1. The transmission path TX has at least one TX filter TXF or a corresponding filter circuit. The reception path RX has at least one RX filter RXF or a corresponding filter circuit. The main purpose of the duplexer is in this case to isolate the TX path and the RX path from one another. This prevents the transmission power of its PA adversely affecting the sensitivity of the receiver. Furthermore, there must be as little attenuation as possible in the respective paths through the duplexer, in order on the one hand to keep the power consumption by the PA low, and in order on the other hand not to unnecessarily adversely affect the signal-to-noise ratio in the reception path.
Nowadays duplexers for mobile telephones are manufactured virtually exclusively using SAW (surface acoustic wave) or BAW (bulk acoustic wave) technology. Both approaches allow component heights which comply with the requirements of flat terminals and, in both approaches, two individual band pass filters are connected to one antenna node. The two individual filters themselves consist of a plurality of suitably connected resonators. In the case of SAW, there may also be one additional DMS track, a plurality of DMS tracks or exclusively one or more DMS tracks, depending on the design (DMS means dual mode SAW). The resonators and/or DMS tracks are each monolithically integrated, and both filters are manufactured on a common substrate, or else on two different substrates. This results in one single chip or two individual chips which are connected to the duplexer, preferably hermetically shielded, in a housing. Furthermore, it is now normal practice to combine one duplexer together with other duplexers and individual filters in a single module on a ceramic substrate, an FR4 substrate or on any other desired substrate. These filters then in each case share one of a plurality of antenna nodes in groups, or else all together share a single antenna node. A duplexer can be interconnected on a module comprising two individual filters, which are each housed individually. A duplexer is also possibly formed through non-housed chips (bare-die) on a suitable substrate.
Further passive components are required for the functionality and for optimization of the electrical behavior of the duplexer, which components are accommodated on the filter substrate or substrates, in the housing, on the module (FEM) or externally on the board in the telephone. In general, these are inductances, capacitances and line pieces. Resonators and suitable electrical (inductive or capacitive) or magnetic couplings are also normal between elements and nodes in the matching network. Furthermore, the ports PA and LNA, must be matched to the desired filter impedance. In the situation with respect to ground, this is 50 Ohms in virtually all cases, and in this balanced case it is 100 Ohms. Series or parallel inductances are normally used in this case, or else L networks comprising an inductance and capacitance. Specifically in the case of duplexers, it is, finally, necessary at the antenna for both filters to have as high an impedance as possible for all frequencies in the respective other band, and to be matched to the required filter impedance for all frequencies in their own pass band. Ideal matching results in a reflection factor Γ of +1 for all frequencies in the other band, and of 0 for all frequencies in their own pass band.
A duplexer consists of a transmission filter and a reception filter. In general, the frequency of the transmission band is lower than that of the reception band. In general, each filter per se is designed such that it is well matched to the antenna port in its own transmission band and is as poorly matched as possible in the respective other band. Each filter then has a pass band in its own band, and a stop band in the other band.
If the two filters were to be connected directly to a common antenna node without any further measures, this then would in general lead to destruction of the pass bands, because of the mutual influence. The reason for this behavior is that, although the condition |Γ|≅1 is satisfied in both cases in the respective other band, the required open circuit Γ≅+1 in the other band will be achieved, however, only in exceptional cases. Normally, a problem of this type is solved by connecting each filter to the antenna via a phase shifter ΦTX or ΦRX.
In this case, the matching curves at the antenna port are each rotated about the center of the Smith chart. In this case, the phase shifters are designed such that the open circuit condition Γ≅+1 is deliberately satisfied by the central rotation in the respective other band of each filter. The matching of its own band remains virtually unaffected by this, because the matching curve for the pass band is only rotated about the origin and |Γ|≅0 is still satisfied. In one simple example, only one phase shifter is required on the RX side, that is to say the TX filter is connected directly to the antenna, and the RX filter is connected via a phase shifter. Other options for matching in a duplexer can be used only for special cases, for example for duplexers with a small duplexer separation, and these are otherwise generally associated with other disadvantages.
Three options are known for providing a phase shifter. A first option is to use a delay line (continuous line). A continuous line rotates in the clockwise direction on the Smith chart and can thus be used as a phase shifter. By way of example, a continuous line such as this is described in U.S. Pat. No. 6,262,637 B1.
The phase-shifting behavior of a continuous delay line can also be provided by a discrete line which can be modeled as a circuit similar to a ladder type, comprising a plurality of inductances and capacitances. In the simplest case, three elements connected in a symmetrical π or T arrangement are used to provide a discrete line. This allows a total of four different configurations. Depending on the configuration, a discrete line such as this rotates in the clockwise direction or counterclockwise direction on the Smith chart.
A further option for a phase shifter can be provided by an antenna coil. By way of example, a transmission or reception filter which ends with a series element at the antenna end and has a suitably capacitive effect in its own pass band is sufficient for phase shifting if the antenna connection can be connected directly with a parallel inductance to ground. This results in a phase shifter similar to a discrete line on both sides of the antenna connection.
Instead of a phase shifter, a parallel coil can also be connected to form a series resonator. A further series coil is optionally used for this parallel circuit, and the filter leads to the antenna node via the circuit created in this way. A circuit such as this can be used by one or both filters. It is particularly suitable for a duplexer when the TX band and RX band have a large duplex separation. This has the disadvantage that, if the duplex separation is small, the series coil requires a high inductance value, which cannot be integrated.